1. Field of the Invention
The invention relates to plasma panel type display screens. It relates more particularly to means for improving the contrast of the image displayed by these screens.
Plasma panels (or PPs) are flat screen display devices working on the principle of the discharge of light in a gas. PPs are used for the display of alphanumerical, graphic or other images, whether monochromatic or polychromatic.
There are different types of PPs, among which it is possible to distinguish those working in continuous mode and those working in alternating mode.
2. Description of the Prior Art
FIG. 1 shows a schematic, sectional view of a standard continuous type PP that can display polychromatic images.
The PP comprises two insulating plates D1, D2 which, between them, demarcate a space 3 filled with a gaseous mixture (the essential component of which is most usually neon). The plates are kept at a distance from each other by thickness shims and a seal (not shown).
According to a commonly used type of organization, each plate D1, D2 bears a network of parallel electrodes. The plates are oriented so that, between the two networks, the electrodes are crossed. Thus, for example, firstly the first plate D1 bears electrodes known as line electrodes Y1, Y2, Y3, Y4 that extend perpendicularly to the plane of the figure and that are seen along their section; to simplify FIG. 1, only four electrodes are shown, but it is usual to find a thousand or more electrodes per network. Furthermore, the second plate D2 bears the second network of electrodes, called "column electrodes" (represented by a single electrode X1) that extends in parallel to the plane of FIG. 1.
Each intersection of a line electrode with a column electrode defines a discharge cell, in such a way that, in the example of FIG. 1, only four cells C1 to C4 are shown, represented by a circle between dashed lines.
The principle of operation is the selective generation (i.e. generation at the level of selected cells) of electrical discharges in gas. Each discharge in gas is accompanied by an emission of light that is localized at each cell in which the electrical discharge is initiated. Each cell may thus constitute an elementary light source whose state (lit or extinguished) can be changed: a figure or a given shape is displayed by lighting up a sequence of cells whose location in the matrix corresponds to the shape of the figure to be displayed.
The color of the light produced by the discharge in the gas depends on the nature of the gas. However, it is common practice to add a light of a different color to this light, so that an observer (not shown) placed on the same side as the first plate D1, called the front plate", perceives a light having the desired color.
To this end, the standard practice is to incorporate one or more photoluminescent elements in the gaseous space 3, the function of these elements being that of converting an ultra-violet radiation, sent out by the discharge in the gas, into a visible radiation of a given color. It is common practice to coat the internal face 4 of the front plate with a homogeneous, photoluminescent layer made of a luminophor or material doped so as to emit at the desired color (in the case of a monochromatic image).
In the case of polychromatic images, the internal face 4 is provided with a succession of photoluminescent elements LB, LV, made of doped luminophors for the different colors which correspond to the so-called primary colors or basic colors used for television. The photoluminescent elements are each placed at the location of a discharge cell to which they give their color. These photoluminescent elements constitute patterns that succeed one another with a repetition that depends on the position assigned to each basic color in a polychromatic pixel PP1, PP2. The term "polychromatic pixel" must be understood to mean a set of discharge cells containing at least two colors.
In the example of FIG. 1, the polychromatic pixels PP1, PP2 are each formed in a standard way by means of four discharge cells:
the first pixel PP1 comprises the first and second cells C1, C2 in which there are respectively positioned a photoluminescent element or luminophor LB for the blue and a photoluminescent element LV for the green. This first pixel PP1 furthermore comprises two other cells (not shown) positioned behind the cells C1, C2 in a deeper plane than that of the figure, one of these cells containing a photoluminescent element for the red and the other cell containing a photoluminescent element for the green. PA1 Similarly, the second polychromatic cell PP2 is formed, firstly, by the third and fourth discharge cells C3, C4 respectively containing a photoluminescent element LB for the blue and a photoluminescent element LV for the green and, secondly, by two other discharge cells in a deeper plane than that of the FIG. 1.
In the example shown in FIG. 1, it can be seen that the photoluminescent elements LB, LV are provided with an aperture 5 facing the line electrodes Y1 to Y4. These apertures 5 are designed to place the line electrodes in contact with the gaseous space in order to further the electrical discharge. It must be noted that these apertures 5 may be made only in the zone located between the surfaces facing the crossed electrodes X1 and Y1 to Y4.
With regard to the alternating type of PPs, they have a memory effect which notably enables the addressing of only the discharge cells for which the "lit" or "extinguished" state has to be extinguished. In panels of this type, the electrodes are covered with a layer of electrical material, and they are no longer in contact with the gas.
Certain alternating type PPs use only two crossed electrodes to define a cell, as described for example in the patent filed on behalf of THOMSON-CSF and published under No. 2 417 848.
There also exist known alternating type PPs, called "coplanar sustaining" electrodes, using three or more electrodes to form a cell. There also exist known alternating type PPs in which all the electrodes are borne by a same plate and are therefore located on a same side with respect to the gaseous space.
The advantage that all these PPs have in common as compared with cathode-ray tubes (or CRTs) is notably that of being highly compact and having flat screens.
However, the PPs having screens with one or more luminophors have the drawback, as compared with CRTs, of having a high coefficient of reflectance that generates an image with insufficient contrast when it is seen in a relatively luminous environment.
In FIG. 1, firstly an arrow EA pointed towards the front plate D1 symbolizes the incident ambient illumination on this front plate and, secondly, a second arrow Lr that emerges from the front plate D1 symbolizes the reflectance. Finally, a third arrow Le symbolizes the intrinsic luminance of the screen (i.e. the luminance of the screen in conditions of zero ambient illumination).
A PP screen or a CRT screen (the latter too has a layer of luminophor material) constitutes, more or less, a scatterer of ambient illumination. Its contrast ratio C=Le/Lr (ratio of intrinsic luminance Le to the backscattered luminance Lr) is practically proportional to the ratio of its intrinsic luminance Le to its coefficient of reflectance r (Le/r).
In view of the similarities between the PP screens and the CRT screens as regards the layers of luminophors, these layers are made with similar technologies in these two types of screen. Consequently, to improve the contrast of the PPs, approaches similar to those of the CRTs are used.
In the case of the CRT, the luminous yield of the tube is sufficient to permit (at the cost of additional energy consumption) approaches that use filtering (neutral or colored), notably by using filters that act both on the intrinsic luminance Le and on the backscattered luminance, until this luminance is greatly reduced.
However, any filtering system induces a loss of luminance, requiring a reserve of light energy to preserve a sufficient dynamic range of luminance. The PPs do not have this reserve of luminous energy, because of their lower luminous yield.
However, trichromatic PP structures, provided with colored filters, are described in the article by TETSUO SAKAI, "A Gas-Discharge Color Panel for TV Display With Ultra-Low Reflectance" (NHK Laboratories Note Ser. No. 380, May 1990). It has been observed that despite notable improvement, the contrast remains far lower than that obtained with a CRT.
At present, in PPs, the layers of luminophor material, namely the photoluminescent elements LB, LV in the example of FIG. 1, are constituted by a thick powdery layer with a thickness E (of the order of 10 microns) that is slightly lower than in the CRTs.
The layer of luminophor material of the elements LB, LV is formed by several monolayers of almost spherical grains G1, G2, G3, . . . , Gn. (A "monolayer" is a layer which has a thickness containing only one grain and is formed by grains that come after one another in a plane substantially parallel to that of the support). The luminophor grains G1 to Gn generally have a mean diameter of the order of 4 micrometers, with a relatively major variation or dispersion of values of the diameter, possibly ranging from 1 micrometer to 30 micrometers.
The inventors have observed that this dispersion of the diameter values notably entails an anarchical positioning of the grains, the result of which is that it is necessary to have a substantial thickness (i.e. a thickness of several monolayers) to obtain a rate of covering that is sufficient to maintain appropriate luminous yield. (The greater the covering rate, the greater is the proportion picked up of ultra-violet radiation emitted by the discharge).
The inventors have also observed that when the thickness of the photoluminescent layer, and hence the number of grains, is increased, there is a tendency to increase the rate of covering (and hence the luminous yield) but that, unfortunately, the coefficient of reflectance r increases at the same time.